The measurement of analytes such as glucose in complex liquid media such as human blood by amperometric methods using disposable test strips has become widely used and is currently employed in a number of commercial products. In certain configurations it is advantageous to improve the signal to noise ratio by employing a three electrode system in which one electrode serves as a pseudo reference/counter electrode to establish a reference potential. Typically this is a silver/silver chloride electrode. A second, working electrode is coated with an enzyme which promotes an oxidation or a reduction reaction with the intended analyte and a mediator which transfers electrons between the enzyme and the electrode. The third xe2x80x9cdummyxe2x80x9d electrode is coated with the mediator but not the enzyme and it provides a measure of the current which arises from other than the oxidation reduction reaction involving the target analyte. An example of such a system is described in U.S. Pat. No. 5,628,980 to Carter, et al. (incorporated by reference herein) and is utilized in the MediSense QID glucose meter.
The three electrode system provides a good way to isolate the current which arises from the oxidation reduction reaction involving the target s analyte such as glucose but it also imposes a higher current load on the pseudo reference/counter electrode. In some testing environments such as glucose meters used by diabetics in their homes it is impractical or impossible to pretreat the samples to remove possible interferants. Thus with home use glucose meters the diabetic simply applies a sample of whole blood. Whole blood typically contains a number of electrochemically active species whose concentration may vary from person to person or even from sample to sample from the same individual. The dummy electrode provides a measure of current arising from the presence of these interferants thus allowing a normalization which removes their contribution to the current measured at the working electrode. However, in such a three electrode configuration the current seen by the pseudo reference/counter electrode includes contributions from both the working electrode and the dummy electrode. Thus in some cases the pseudo reference/counter electrode sees a significantly greater current than it would in a two electrode configuration.
The pseudo reference/counter electrode in such a configuration is, in act, serving two roles which can be inconsistent if the current it sees becomes too great. It serves, on the one hand, to provide a constant half-cell potential, i.e. a reference potential and, on the other hand, it also serves as a counter electrode balancing the electron transfer occurring at the working and dummy electrodes. For instance, in a typical glucose meter,mediator is becoming oxidized at the working and dummy electrodes so a reduction reaction needs to occur at the pseudo reference/counter electrode to balance the electron transfer. With the typical Ag/AgCl pseudo reference/counter electrode this involves the reduction of silver ions thus consuming (or reducing) silver chloride. If too much silver chloride is consumed the pseudo reference/counter electrode can no longer serve its function of providing a source of constant half-cell potential. In other words, the potential difference between the two electrode reactions such as the oxidation of a mediator at the working electrode and the reduction of silver at the pseudo reference/counter electrode will actually shift as the reaction proceeds.
One approach is to redesign the pseudo reference/counter electrode to handle higher current loads without displaying a significant shift in half-cell potential. This would normally mean increasing the size or silver concentration of the pseudo reference/counter electrode relative to the working and dummy electrodes. It is difficult to further reduce the size of the working electrode because its size has already been minimized. It is limited by the economically acceptable procedures for reproducibly manufacturing millions of such disposable test strips. On the other hand, increasing the size or silver concentration of the pseudo reference/counter electrode would significantly increase the cost of such three electrode disposable strips because silver is the most expensive material used in the construction of such strips.
Therefore, there is a need for three electrode disposable test strips for use in amperometric systems whose cost is comparable to two electrode test strips and yet have pseudo reference/counter electrodes with about the same stability as in the two electrode test strips.
It has been discovered that the current load on the pseudo reference/counter counter electrode in a disposable test strip for use in amperometric measurements with a three electrode system can be decreased and therefore its half cell potential better stabilized by increasing the resistance of the dummy electrode. This allows three electrode test strips to give better performance without changing the operating characteristics of the meters in which they are used.
Increasing the resistance of the dummy electrode not only reduces the total current passing through the pseudo reference/counter electrode but it also changes the potential at the dummy electrode""s interface with the sample. Thus it is possible to have a three electrode system which can simultaneously measure the concentration of two analytes. The effective potential at the xe2x80x9cdummyxe2x80x9d electrode with the higher total resistance can be adjusted to be too low to effect an oxidation reduction reaction indicative of the concentration of one of the two target analytes.
It is preferred to have the resistance of the dummy electrode be at least 1000 ohms greater than that of the working electrode and it is especially preferred that the resistance differential be at least about 4000 ohms.
It is also preferred that the resistance of the dummy electrode be increased by putting a resistance in series with the active electrode surface of this electrode. Thus both the area and nature of the active surface of the dummy electrode are kept similar or identical to that of the working electrode. This can readily be achieved by increasing the resistance of the conductive track which connects the active electrode surface to the meter which applies the potential and measures the resulting current. In the typical disposable strip for amperometric analyte measurement three electrode surfaces are present on one end of an elongated flat strip and three contact pads, one for each of the electrode surfaces, are present on the other end of the strip. Each electrode surface is connected to its contact pad by a conductive track. The contact pads serve as the means to establish electrical contact between the strip and the meter which applies the potential and measures the resultant current. The conductive tracks are typically covered by an insulating layer to prevent any short circuits between them.
It is particularly preferred to increase the resistance of the conductive track of the dummy electrode by narrowing its width. If this conductive track is made of the same material as the working electrode""s conductive track and has about the same thickness as the conductive track of the working electrode it will have a higher resistance. Such a mechanism of increasing resistance is particularly easy to implement in mass manufacturing.